The uptake of sulfate by Lemna has been studied. Two systems have been demonstrated. One has high affinity for sulfate (Km approx. 9 MicroM) and a Vmax of 2.7 nmole/frond x day. This system is dramatically down-regulated by prior growth of Lemna in either cystine or high concentrations of sulfate. A second uptake system has an affinity for sulfate so low as not to be saturable at tolerable concentrations of sulfate (up to 25 mM). At 25 mM sulfate, the uptake via this system is 7-10 nmole/frond x day. This system is down-regulated little, if at all, by prior growth of the plants in cystine or high concentrations of sulfate. Neither system is inhibited by phosphate or nitrate at the concentrations in normal growth medium. The temperature dependence of the high affinity system is more marked than is that of the low affinity system.